v3d: Fix incorrect handling of two fences created back-to-back.

Recreating our context's syncobj with ALREADY_SIGNALED meant that if you
created two fences in a row, then waiting on the second would succeed
immediately. Instead, export a sync file in the gallium fence (since we
don't have a syncobj clone ioctl), and just create a new syncobj to wait
on whenever we need to.
Noticed while debugging
dEQP-GLES3.functional.fence_sync.client_wait_sync_finish